1. Field of the Invention
The present invention relates to a self-advancing screw suitable for fastening of a plasterboard. In the following description, the self-advancing screw will be simply referred to as the “screw” in many cases.
2. Description of the Related Art
The self-advancing screws are broadly divided into a narrow-tip type having a pointed tip end and a drill type in which a drill portion having a cutting edge is provided to a tip end, in terms of the shape of the tip end portion. The former narrow-tip-type screw is typified by a wood screw. The narrow-tip-type screw spreads out tissue of a member with its tip end portion to allow its shaft to advance into the member, and is often used for fastening various kinds of workpieces to a base made of soft material such as wood or a base made of a thin steel sheet of a thickness of about 0.5 to 0.8 mm.
The drill-type screw is often used for fastening various workpieces to a base made of a steel sheet in general, e.g., fastening a workpiece made of a steel sheet to a base made of a steel sheet or fastening a wooden workpiece to a base made of steel. It is needless to say that the screw is also used for fastening a workpiece to a soft material such as a wooden base (see Japanese Patent Application Laid-open No. 2002-31115, for example).
For both the types of screws, a variety of improvements have been proposed conventionally. For example, with regard to the narrow-tip-type screw, two screw threads are formed on a shaft, and many variations have been proposed based on this double-thread type. Examples of the variations are as follows.
For example, there is a screw disclosed in Japanese Patent Application Laid-open No. 2001-200822. In the screw the whole thread portion of which is formed of two screw threads, a certain area of the thread portion located on a head side is formed as the same height portion in which the two screw threads have the same heights (or outside diameters) and a portion of the thread portion located on a tip end side of the same height portion is formed as a different height portion in which the two screw threads have different heights (or outside diameters).
There is also a screw disclosed in Japanese Patent No. 3334563, in which, in the case where the whole thread portion is formed of two screw threads, the thread portion is divided into three parts, i.e., a small-diameter parallel thread portion located on a tip end side, a large-diameter parallel thread portion located on a head side, and a tapered intermediate thread portion smoothly connecting the small-diameter parallel thread portion and the large-diameter parallel thread portion.
There is also a screw disclosed in Japanese Patent No. 3413828, in which, in the case where the whole thread portion is formed of two screw threads, the thread portion is divided into three parts, i.e., a straight lower thread portion located on a tip end side, a straight upper thread portion located on a head side, and an intermediate thread portion located between the lower thread portion and the upper thread portion. Moreover, the relationship between outside diameters of the thread portions is set to be “the outside diameter of the lower thread portion<the outside diameter of the intermediate thread portion<the outside diameter of the upper thread portion”.
There is also a screw disclosed in Japanese Patent No. 3487210. In the case where the whole thread portion is formed of two screw threads, a certain area of the thread portion on a tip end side is formed as a small-diameter portion in which screw threads are extremely low, and a portion of the thread portion located on a head side of the small diameter portion is formed as a large-diameter portion in which screw threads are high.
Furthermore, a screw disclosed in Japanese Patent No. 3060284 includes a single-thread portion located on a tip end side and a double-thread portion located on a head side of the single-thread portion. An outside diameter of the single-thread portion is set of a smaller dimension than an outside diameter of the double-thread portion, and a height of a screw thread of the single-thread portion is set to be much smaller than heights of screw threads of the double-thread portion.
There are screws disclosed in Japanese Patent Application Laid-open No. 60-23614 and Japanese Patent Application Laid-open No. 9-273526. In each of the screws, a shaft has different diameters, i.e., diameters of a small-diameter portion located on a tip end side and of a large-diameter portion located on a head side. One screw thread extends throughout the small-diameter portion and the large-diameter portion.